The present invention relates to a variable venturi-type carburetor.
The inventor has proposed a variable venturi-type carburetor having a raised wall which projects from the inner wall of the intake passage, which inner wall faces the tip face of the suction piston serving to change the cross-sectional area of the venturi portion of the carburetor in response to a change in the amount of air fed into the cylinder of the engine. In this carburetor, a D-shaped air inflow mouth is formed between the substantially straight extending tip edge of the raised wall and the substantially circular cross-section inner wall of the intake passage, and at least the upstream end portion of the tip face of the suction piston is shaped in a V-shaped cross-section which expands toward the venturi portion so that an approximately triangular-shaped air inflow opening is formed between the tip edge of the raised wall and the upstream end portion of the tip face of the suction piston when the amount of air fed into the cylinder of the engine is small. In this carburetor, as mentioned above, the air inflow opening has an approximately triangular shape when the amount of air fed into the cylinder of the engine is small so that the opening area of the air inflow opening is as proportional to the lift of the suction piston as possible in order to obtain the smooth movement of the suction piston when the amount of air fed into the cylinder of the engine is increased. However, in this carburetor, when the amount of air fed into the cylinder of the engine is small, since the cross-sectional area of the venturi portion formed at a position located downstream of the raised wall is considerably larger than the area of the above-mentioned triangular-shaped air inflow opening, air flowing into the venturi portion from the air inlet opening is decelerated in the venturi portion. This reduces the velocity of the air flowing around the nozzle, thereby making it difficult to sufficiently promote the vaporization of fuel fed from the nozzle.